There are many factories and plants, especially nuclear power plants, in which many items of equipment are installed in containment buildings intended to prevent the spread of contaminants, radioactivity and the like in the event of an accident. These plants include numerous control devices, such as motor operated valves, which must be tested periodically to insure reliable operation, especially during emergency conditions. Thus it becomes necessary to periodically shut the plant down to allow access to the containment area for personnel to test this equipment. In order to minimize plant downtime, especially for auxiliary equipment testing, and to reduce maintenance cost, various proposals have been made to provide permanent diagnostic equipment, such as position sensors, strain gages, flow meters and the like which remain attached to the various control devices, and to provide cabling to a central data acquisition point. For example, a data acquisition point may be provided in the plant control room, so that testing may be performed outside of the containment area or continuous monitoring performed during normal operation. However, the usual problem in these types of plants is that there is no unused cabling available from the devices to the control room or other remote location, and it is usually too expensive to install new electrical cabling. Further, in nuclear plants it is difficult to obtain authorization to install additional cabling through the containment enclosure. Therefore, while it may be desirable to install permanent data acquisition instrumentation on certain control devices within the plant, the high cost of installing the needed cabling usually prevents it.
Thus, there is a need for a means of transmitting signals from diagnostic equipment and other information-carrying signals over existing electrical cables, usually noisy AC power lines which are already connected to the various control devices. An investigation of commercially available systems to transmit signals over AC power lines revealed that there were no devices available capable of transmitting low frequency (0-1000 Hz) analog information over electrical lines already used for other purposes, such as AC power lines operating at 50 or 60 Hz.
Digital technology has been applied for networking microcomputers with all kinds of home appliances over existing AC power lines, telephone lines, radio or infrared optical links, etc. A discussion of this type of signal transmission may be found in an article entitled "Control Networks for the Home" by F. W. Gutzwiller, published in Machine Design, Oct. 20, 1983, p. 109-112.
Systems are available for limited voice frequency transmission over AC power lines using frequency modulation/demodulation technology for analog signals in the range of from about 500 Hz to 5 kHz. Examples of these types of systems are the Three Channel FM Intercom, Catalog No. F-1010, available from Dick Smith Electronics, Inc., P.0. Box 8021, Redwood City, Calif. 94063; and the Radio Shack, Model No. 43-218 Intercom available from Radio Shack Corp., Fort Worth, Tex. 76102.
None of the above systems are capab1e of transmitting low frequency analog signals down to DC (0 Hz). To provide a reliable signal transmission system for transmitting signals from diagnostic instrumentation there is a need for an analog transmission system which is capable of transmitting low frequency analog signal information, down to DC, over noisy AC power lines.